In the composition of a gasoline pool, the olefinic gasolines issued from cracking, visbreaking or coking units, form a substantial part of motor gasolines. However, generally these olefinic gasolines have relatively low octane numbers, and it has been suggested to increase their octane number by etherification of certain of their olefins with methanol. As a matter of fact, the light fraction of these olefinic gasolines contain tertiary olefins such as isoamylenes, isohexenes and isoheptenes which may easily react with alcohols such as methanol to give methyl ethers.
This etherification is generally performed with acid catalysts and more particularly with ion-exchange resins such as sulfonic resins.
The treatment of these gasolines with methanol in the presence of sulfonic resins provides an increase in their octane number and, in addition, decreases their olefin content, mainly tertiary olefin content, making it possible to upgrade methanol to premium gasoline without the problems arising when methanol is directly introduced in the gasolines, increases in the vapor pressure of the gasoline and phase separation when water is introduced in the storage tanks and the distribution circuits.
It is for this last reason that regulations in many countries do not allow the introduction of free methanol in gasolines except in the presence of co-solvents which eliminate separation problems.
The reaction between methanol and tertiary olefins is balanced and it is generally difficult to obtain conversion rates close to 100%. The conversion rate of the tertiary olefins is further reduced as the molecular weight of the olefin is increased.
Thus, whereas conversion of isobutene may be as high as 93 to 98% by mere passage over the catalyst, the conversion rate of isoamylenes, as described in French Pat. No. 2 411 881, in conditions similar to those used for producing methyl-tert butyl ether (MTBE), are at most about 50 to 60%. This patent shows that by recycling to the reactor a part of the light fraction containing the unreacted isoamylenes obtained by distillation of the reactor effluent, it is then possible to obtain a maximum conversion rate of about 70%, under operating conditions, reasonably economical in view of the high cost of the distillation.
Such a process flowsheet does not provide for the removal of the methanol excess remaining in the reactor effluent and which is hence found in major part in the free state in gasoline.
In order to completely remove the methanol excess contained in the mixture obtained by etherification of a mixture containing C.sub.4 and C.sub.5 tertiary olefins, it has been suggested, in the U.S. Pat. No. 4,204,077, to remove the methanol from the reactor effluent by extraction with such a solvent as ethylene glycol.
The authors of said patent consider that this solvent is preferable to water, which, during methanol extraction, drives along therewith a portion of the ethers contained in the reactor effluent.